This invention relates generally to golf balls, and more particularly to wound golf balls that have radially-extending sections of thread windings with variable stress, and a method for forming such golf balls.
Wound golf balls are the preferred ball of more advanced players due to their spin and feel characteristics. Wound balls typically have either a solid rubber or fluid-filled center around which a wound layer is formed, which results in a wound core. The wound layer is formed of thread that is stretched and wrapped about the center. The wound core is then covered with a durable-cover material, such as a SURLYN(copyright) or similar material, or a softer xe2x80x9cperformancexe2x80x9d cover, such as Balata or polyurethane.
Wound balls are generally softer and provide more spin than solid balls. This enables a skilled golfer to have more control over the ball""s flight and final position. Particularly, with approach shots into the green, the high spin rate of soft-covered-wound balls enables the golfer to stop the ball very near its landing position. In addition, wound balls exhibit lower compression than two-piece balls. Their higher spin rate means wound balls generally display shorter distance than hard-covered-solid balls. The advantages of wound constructions over solid ones, however, relate more to targeting or accuracy than distance.
To meet the needs of golfers with various levels of skill, golf ball manufacturers also vary the compression of the ball, which is a measurement of the deformation of a golf ball under a fixed load. A ball with a higher compression feels harder than a lower-compression. Wound golf balls generally have a lower compression than solid balls, which is preferred by better players. Whether wound or solid, all golf balls become more resilient (i.e., have higher initial velocities) as compression increases. Players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact; therefore, manufacturers of both wound and solid golf balls balance the requirement of higher initial velocity from higher compression with the desire for a softer feel from lower compression.
To make wound golf balls, manufacturers use automated winding machines to stretch the threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. As the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more-lively wound ball is produced, which is desirable.
Some methods attempt to employ constant tension during the entire winding process by attempting to apply a constant pull or force on the thread. However, variations in thread cross-sectional area prevent balls formed under constant pull from having constant stress or constant elongation throughout the ball. For example, as the cross-sectional area of the thread decreases, the thread stretches to a greater degree given a constant pull. Conversely, as the cross-sectional area of the thread increases, the thread stretches to a lesser degree under a constant pull. This results in uncontrolled variations in stress and in compression throughout the finished ball, which may negatively affect the ball""s performance.
Furthermore, to account for variations in thread cross-sectional area, manufacturers of wound balls do not wind using the maximum tension or stretch the thread to the maximum elongation, because to do so would cause an excessive amount of thread breakage during manufacture or play. This also prevents manufacturers from optimizing ball performance. In addition, the rubber elastic modulus also affects compression which is not considered when manufacturers attempt to control tension alone.
U.S. Pat. No. 4,783,078 to Brown et al. discloses one method used in an effort to decrease thread breakage. In this patent, thread is wound first at low tension then at high tension. Controlling tension alone, however, is an approximate means of achieving the desired compression.
U.S. Pat. No. 2,425,909 to Wilheim discloses one winding method that considers the cross-sectional area of the thread during winding. In this patent, an apparatus measures the pounds per square inch tension of a portion of thread during winding, applies the level of total tension to the thread during winding, and automatically adjusts the level according to the pounds per square inch tension measurement in order to keep the pounds per square inch tension value constant throughout the winding process.
Golf ball manufacturers are continually searching for new ways in which to provide wound golf balls that deliver improved performance for golfers while decreasing the occurrence of thread breaks both during manufacturing and during play. It would be advantageous to provide a wound golf ball with a higher compression, higher initial velocity, improved durability, and improved manufacturing processibility. The present invention provides such a wound golf ball.
The present invention is directed to an improved golf ball and a method of winding a golf ball that includes measuring and controlling thread stress directly, rather than maintaining and controlling the level of tension on the threads.
The golf ball includes a center, a wound layer that surrounds the center to form a wound core, and a cover that surrounds the wound core. The wound layer is formed of at least one thread, and the wound layer includes a plurality of radially-extending sections. Each section has a thread stress. The stress within each section is substantially constant, but at least two radially-extending sections have different stresses. The thickness of a radially extending section can be as small as about 0.007 inches. According to one aspect of the present invention, substantially constant means the percentage stress variation within a section is less than the percentage thread cross-sectional area variation within the same section.
In one embodiment, the stresses are different by at least 10%. In another embodiment, the stress increases from one radially extending section to another in a radially-outward direction. In another embodiment, the stress decreases from one radially-extending section to another in a radially-outward direction. The stress can also alternate between sections.
In one embodiment, the golf ball further includes first and second stresses. The first stress is less than about 40% of the breaking stress of the thread and the second second stress is greater than about 40% of the breaking stress.
In yet another embodiment, the golf ball can include first- and second-wound layers, and a molded-intermediate layer. The first-wound layer surrounds the center. The molded-intermediate layer surrounds the first-wound layer, and the second-wound layer surrounds the first-wound layer. The first- and second-wound layers include a plurality of radially-extending sections, each section has a stress, wherein at least two sections within each layer have different stresses and the stress within each first section is substantially constant.
According to one aspect of the present invention, the stress varies constantly or in intervals.
The method for winding thread onto a golf ball center to form a wound core comprises the steps of measuring stress within a portion of the thread; winding the thread about the golf ball center while applying a force thereon to form a first portion with a predetermined thickness and the first portion having thread with stress equal to a first value; and winding the thread about the golf ball center under the force to form a second portion with a predetermined thickness and the second portion having thread with stress that is different from the first value.
In one embodiment, winding the thread about the golf ball center to form the second portion further includes forming a plurality of subsections within the second portion of predetermined thickness. The stress in the subsections varies from the first value and varies from the stress value in adjacent subsections.
In another embodiment, the stress varies during winding constantly or in intervals.
According to one aspect of the present invention, the step of measuring the stress is continuous during winding and further includes passing the thread over at least two-spaced rollers; vibrating the portion of the thread between the rollers; measuring the vibration of the portion of the thread; and calculating stress from the vibration measurement. The step of vibrating the portion of the thread between the rollers further includes directing air upon the portion of the thread.
According to yet another aspect of the present invention, the step of applying the force further includes passing the thread over a tension wheel that rotates about a shaft, and applying a braking force on the shaft. For example, a magnetic brake or a friction brake applies the braking force.